The autopilot’s vertical speed mode allows you to perform constant-rate climbs and descents. Figure 4-10 illustrates the use of the vertical speed mode for one autopilot that is integrated with a PFD.
When you engage the vertical speed mode, the FD/autopilot will attempt to maintain the specified vertical speed until you choose a different setting in autopilot, the aircraft reaches an assigned altitude set into the assigned altitude selector/alerter, or the autopilot is disconnected. If an altitude selector is not installed or functioning, the pilot has the task of leveling off at the assigned altitude, which requires monitoring progress and manually engaging the autopilot’s altitude hold function once the aircraft reaches the desired altitude. You must be very careful to specify an appropriate vertical speed, as the aircraft will fly itself into a stall if you command the autopilot to climb at a rate greater than the aircraft’s powerplant(s) is/ are capable of supporting. You also need to monitor descent airspeeds diligently to ensure compliance with VNE/VMO and VA or turbulence penetration speeds if there is doubt about smooth air conditions. As discussed in the previous chapter, you should be cognizant of the powerplant temperatures reciprocating powered aircraft and bleed air requirements for turbine-powered aircraft.
Vertical Speed With Altitude Capture
Some FD/autopilots have an altitude select/capture feature. The altitude select/capture feature is illustrated in Figure 4-11. The altitude select/capture feature combines use of the activated vertical speed mode and an armed altitude hold mode. To use this feature, the vertical speed function is initially engaged. The altitude hold mode usually arms automatically when a different altitude is selected for capture and vertical speed is activated. With an altitude select/capture option or feature, the altitude hold mode disengages the vertical speed mode upon capture of the selected altitude once the vertical speed function completes the necessary climb or descent. Once the aircraft reaches the assigned altitude, the vertical speed function automatically disengages, and the altitude mode changes from armed to engaged. The change from vertical speed mode to altitude hold mode is the capture mode, or transition mode. Any changes made by the pilot during this short phase usually result in a cancellation of the capture action, allowing the aircraft to continue the climb or descent past the selected altitude. Again, be familiar with the aircraft’s equipment. Let the system complete programmed tasks, and understand what it will do if interrupted.
Many FD/autopilot altitude selectors include an altitude alert feature, an auditory alert that sounds or chimes as the aircraft approaches or departs the selected altitude.
Catching Errors: Armed Modes Help Prevent Forgotten Mode Changes
You have already seen how remembering to make a needed mode change in the future can be an error-prone process. Not canceling the armed function allows the altitude select mode to relieve the pilot from needing to remember to engage the function manually once the aircraft has reached the selected altitude. Do not interrupt the altitude armed or capture mode, unless prepared to manually control the process.
The indications on the autopilot in Figure 4-11 do not distinguish between functions that are armed or engaged.
The more sophisticated annunciator shown in Figure 4-12 uses color coding to distinguish between armed and engaged autopilot functions.
Common Error: Failure To Arm the Altitude Mode
The most common error made by pilots during climbs and descents is failure to arm the altitude mode to capture the assigned altitude. In many instances, this happens when the crew does not correctly adjust the altitude selector or alerter. Sometimes, this malfunction occurs when the altitude is adjusted at the same time the system is attempting to go into the capture mode. This situation typically results in the aircraft climbing or descending beyond the assigned altitude, which may result in an altitude deviation. Altitude deviations are among the most common mishaps reported by pilots to NASA’s Aviation Safety and Reporting System (ASRS). In any event, always monitor the actions of the FD/autopilot system and be prepared to fly the aircraft manually.
Awareness: Altitude Alerting Systems
Altitude alerting systems were mandated for commercial jet transports in the early 1970s in response to a growing number of altitude deviations in airline operations. Although they helped reduce the total number of altitude deviations, altitude alerting systems also made possible a new kind of error. Altitude deviation reports submitted to the Aviation Safety and Reporting System (ASRS) indicate that pilots sometimes rely too much on the altitude alerting system, using it as a substitute for maintaining altitude awareness. Instead of monitoring altitude, pilots sometimes simply listen for the alert. This phenomenon is one instance of what human factors experts call primary-secondary task inversion—when an alert or alarm designed as a secondary backup becomes the primary source of information. In the case of the altitude alerting system, when the alerting system is missed, or you are distracted, nothing is left to prevent an altitude deviation. You must remember that the altitude alerting system is designed as a backup, and be careful not to let the alerting system become the primary means of monitoring altitude. Most airline operators have a standard operating procedure that requires pilots to call out approaching target altitudes before the altitude alerting system gives the alert. Common errors occur when setting 10,000 feet versus 11,000 feet. Too many ones and zeros can confuse a fatigued, busy pilot, resulting in setting an incorrect altitude.
Awareness: Automatic Mode Changes
Distinguishing between “armed” and “engaged” adds complexity to the process of maintaining mode awareness. In addition to autopilot functions that are engaged by the pilot, some autopilot functions engage and disengage automatically. Automatic mode changes add to the challenge of keeping track of which autopilot functions are currently engaged and which functions are set to become engaged. You can minimize confusion by always verifying the status annunciations on the FMs, PFD/MFD, and the autopilot mode annunciator after any change of heading, altitude, or vertical speed. The verification process forces you to carefully consider the configuration of the FMS and FD/ autopilot. Determine if engaging the autopilot cancels certain FD modes. Some units interact, and when the autopilot is engaged, some FD modes are automatically canceled, notably altitude hold or selection.
Learning: The Importance of Understanding
One way to learn the steps required to use an autopilot is simply to memorize them. This approach focuses solely on the button and control manipulations required to perform each procedure. Although this approach to learning may appear to be the quickest, studies have shown that pilots who take the time to develop a deeper understanding of how a system works give themselves three important advantages. These pilots are better able to:
- Work through situations that differ from the ones they learned and practiced during training,
- Transition from one manufacturer’s system to another, and
- Recall procedures after not having practiced them for some time.
Investing time to understand FD/autopilot functions pays off. For example, in many systems, once the aircraft reaches the selected altitude and levels off as indicated by the altitude mode annunciator, the pilot can select the next altitude in the window. Then, upon receiving the clearance to climb or descend, the pilot must select only the vertical mode. In many systems, the vertical speed mode is indicated and the altitude mode is indicated as “armed” and ready to capture the selected altitude. Only the power requires pilot manual control.
Unless the aircraft has an autothrottle system, you must adjust the power to an appropriate setting when performing any climb, descent, or level-off. You cannot allow the aircraft to exceed any applicable speed limitations during a descent. During a climb at a vertical speed that the aircraft cannot sustain, the FD/autopilot may command a pitch that results in a stall.
- Use the FD/autopilot to climb or descend to and automatically capture an assigned altitude.
- Determine the indications of the armed or capture mode, and what pilot actions will cancel those modes.
- Determine if the system allows resetting of the armed or capture mode, or if manual control is the only option after cancellation of these modes.
- Determine the available methods of activating the altitude armed or capture mode.
- Determine the average power necessary for normal climbs and descents. Practice changing the power to these settings in coordination with making the FD/ autopilot mode changes.
- Determine and record maximum climb vertical speeds and power settings for temperatures and altitudes. Ensure the values are in agreement with values in AFM/POH for conditions. Make note of highest practical pitch attitude values, conditions, and loading. Remember powerplant factors (e.g., minimum powerplant temperature, bleed air requirements) and airframe limitations (e.g., VA in setting power).